Structural characterization of mullites synthesized by thermal decomposition of topaz

Abstract

The structure of mullite obtained by thermal decomposition of topaz at 1340 °C and after additional thermal treatment at 1600 °C was characterized by 29Si and 27Al MAS NMR spectroscopy and x ray diffraction. No amorphous phase was detected by these techniques. The 29Si MAS NMR spectra of the mullite samples showed four resonance peaks at -81, -86, -90 and -94 ppm. The peak at -81 ppm corresponded to silicon near oxygen vacancies in the mullite structure. The peak at 86 ppm was related to a sillimanite-type site and was the major contribution, ranging from 42 to 50%. The two other peaks, at -90.0 ppm and -94 ppm, were interpreted as resulting from rearrangements of the sillimanite-type site by the replacement of AlO4 by SiO4 tetrahedra16. The 27Al MAS NMR data of the two samples exhibited three different peaks, two of which were attributed to tetrahedral sites at 40 -45 and 63 -67 ppm, while the third was assigned to an octahedral site at about -6 ppm. The ratio of tetrahedral to octahedral aluminum sites was found to depend on the temperature applied during the preparation of the mullite and was higher at 1600 °C

Characterization of porcelain tableware formulation containing bentonite clay

“…Mass loss associated with these peaks was about 13.88, 13.36, 12.22 and 13.78% for 0, 5, 10 and 15 wt.% bentonite, respectively. Endothermic peaks at 526.1, 519.3, 538.1 and 515.1°C for 0, 5, 10 and 15 wt.% bentonite, respectively, were assigned to the dehydroxylation of the hydroxyl groups in the kaolin to form metakaolin (A1 2 O 3 , 2SiO 2 ) (Schneider et al, 1994) and dehydroxylation of montmorillonite (Celik, 2010). Mass loss associated with these peaks was 3.78, 3.67, 3.70 and 3.58% for 0, 5, 10 and 15 wt.% bentonite, respectively.…”

Luminescent Eu3+ doped Al6Ge2O13 crystalline compounds obtained by the sol gel process for photonics

“…The resulting 126-atom supercell for mullite consists of 36 Al, 12 Si, and 78 O atoms [20], being~16 units of AlO 6 octahedra and~20 units of AlO 4 tetrahedra. However, the tetrahedral to octahedral aluminum site ratio is dependent on the heat-treatment temperature [15] and certainly on the synthesis process.…”

“…As a consequence, it can be assumed that Eu 3þ should prefer replace Al 3þ of AlO 6 octahedra (designated as M sites), because they have same valence and smaller difference between its ionic radius, also the octahedral sites can accommodate easily lanthanide elements than tetrahedral sites. Note that in octahedral sites of mullite the average interatomic distance between Al and O atoms is 1.913(2) Å, and in tetrahedral sites is 1.700(2) Å [15], and consequently some octahedral sites can well accommodate Eu 3þ ions. Fig.…”

“…In the crystallization process, the mass of the sample reduces around 1% was related to an elimination of carbon and/or hydroxyl groups strongly linked to the metals, followed by a gradual increasing of the mass probably due to oxygen absorption from atmosphere (residual oxidation reaction) and some broad exothermic peaks related to changes in the AlO 4 e AlO 6 groups ratio in the crystalline phase (to be confirmed in future works for this phase). Some works on mullite (3Al 2 O 3 :2SiO 2 ) studies mention that the relation between tetrahedral and octahedral groups is dependent of annealing temperature [15]. [17] produced Al 6 Ge 2 O 13 ceramic powder by the co-precipitation method, using Al(NO 3 ) 3 and Cl 3 GeCH 2 CH 2 COOH as precursors.…”